A form of an information storage and retrieval device is a hard disk drive (hereinafter “disk drive”). A disk drive is conventionally used for information storage and retrieval with computers, data recorders, redundant arrays of independent disks (RAIDs), multi-media recorders, and the like. A disk drive comprises one or more disk media.
Each disk medium comprises a substrate upon which materials are deposited to provide a magnetically sensitive surface. In forming a disk medium, a substrate is ground or polished, conventionally by chemical-mechanical or mechanical polishing, to provide a substantially planar surface. Layers of materials are substantially uniformly deposited on this substantially planar surface to provide magnetic properties for writing to and reading from the disk media.
However, defects, such as pits, voids, particles, bumps and scratches, among others, may arise on a disk medium surface. These defects affect the surface topography of the disk medium. These defects need to be detected and characterized. A number of different types of apparatus can be used to measure the surface topography. These include a Candela profilometer, a quadrature phase shift interferometer, or a laser doppler vibrometer, for example.
In particular, a quadrature phase shift interferometer is designed to provide an optical, non-contact testing method for inspecting a disk medium surface, or other ultra-smooth surface. Defects are detected and characterized by out-of-plane displacement. The interferometer described is able to measure out-of-plane displacements with nanometer resolution with frequency responses in a range of DC to hundreds of megahertz depending on detector rise time.
Phase angle calculation is an intermediate goal of a QPSI decoding algorithm. The ultimate goal of the QPSI is to measure out-of-plane displacement, which can be related to the phase angle through fundamental and well-known constants. One of the known methods used to resolve phase angle is the maximum/minimum intensity method. To employ this method, an accurate intensity envelope of the signal is required. A simple method to detect the intensity envelope uses the location of peaks and valleys within the signal. Curves are fitted through these peaks and valleys to create an intensity envelope. There are several peak/valley detection methods. The most commonly used method is the peak detector method. This method is based on an algorithm that fits a quadratic polynomial to sequential groups of data points from the signals produced by the detector. The peak detector method searches for a zero crossing in the first derivative of the signal in conjunction with the condition that the signal satisfies a threshold criterion at the location of the derivative zero crossing. However, referring now to FIG. 1, the method is easily confounded by false peaks and valleys that are present in the detected signals. In FIG. 1, a typical set of I and Q signals are depicted with modulated amplitude and corresponding ideal intensity envelopes.
According to conventional peak detection methods, the points a, b and c in FIG. 1 can be mistakenly identified by the peak detector as maximum and minimum intensity points, although they are not true maximum or minimum intensity points. Since the signal envelope will not be correctly identified, due to these false maximum and minimum intensity points, a significant decoding error will result.